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Re: Spare ribs?



In a message dated 2/14/01 1:38:56 AM EST, mbonnan@hotmail.com writes:

<< Chevrons are not homologous to ribs, however, because they are essentially 
modified haemal arches and arise below (ventral to) the bodies (centra) of 
the vertebrae.  Ribs articulate with the centra and transverse processes 
instead. >>

If you look at the skeletons (such as they are) of the most primitive living 
vertebrates, it seems to me that the cervical ribs, thoracic/abdominal ribs, 
and hemal arches form a pretty continuous series along the vertebral column, 
and that the details of how and where they articulate with the vertebrae in 
various places along the column are secondary. This leads me to suspect that 
they arise in the same way from the same precursor structures during the 
ontogenetic development of the vertebrate body. Unfortunately, my copy of 
Romer's Osteology of the Vertebrates is buried, so I cannot confirm this (if 
this topic is even discussed there); but it seems unparsimonious for 
vertebrates to develop ribs in one set of loci and hemal arches independently 
in another set of loci where the ribs are not found. The only other thing I 
can think dinosaurian hemal arches might be is modified intercentra, which 
seems possible but weird.

Any embryologists out there who might shed some light on this?